Issues in Window Selection: Human Factors
Daylight | Glare | View | Thermal Comfort
Glare
Too much daylight can produce excessive glare, which is particularly undesirable in computer work environments. Glare results when the source of the light is too intense for the naked eye to handle. As the eye attempts to even out the contrast between the task and the surrounding surfaces, the muscles of the eye have to work harder and more frequently. Tired eyes and increased levels of stress result. Glare within the range that the eye can handle is called discomfort glare; glare preventing us from doing a task is called disability glare. In addition to these two glare categories, there is direct and indirect glare. Direct glare is caused when a person views the source of illumination. Indirect glare results from light being reflected off surfaces.
Like the strain associated with glare, the eye has a difficult time adapting to high brightness ratios between differently illuminated tasks within one's field of view. Because brightness is a function of reflectance and illumination, the brightness ratio is controllable through good design. Anatomically, the eye is more sensitive in brightness ratios at the center of the field of vision, but brightness ratios in the periphery nevertheless invoke a reflex to center the eye on the brightness difference. Thus, changes in brightness ratios due to daylighting or artificial lighting need to be kept low over large areas of an occupant's field of view in a space. By keeping the reflectance of wall surfaces within the levels shown in Figure 2-38, excessive brightness ratios can be minimized. Direct sun also must be controllable. The Illuminating Engineering Society (IES) recommends that small patches of sunlight be controlled to less than 79 candelas per square foot.
To provide a general indication of the potential glare problem with different windows, the DOE-2.1E program calculates a daylight glare index at a specified location and orientation of a viewer within the space. This glare index is based on a subjective response to brightness within one's field of view. In this analysis, the average annual glare index is computed for a person facing one of the side walls sitting 5 feet from the window. A glare index of 10 is the threshold for just perceptible glare, while a glare index of 16 is the threshold where glare is just acceptable. The threshold for just uncomfortable glare and the maximum value recommended for general office tasks is 22. Figure 2-39 shows the average glare index in a south-facing perimeter office in Chicago. The lowest glare index occurs with Window D, which has a very low VT of 0.10. None of the windows has an average glare index over 10 even though the window is unshaded.
Similar to daylighting, the average annual glare index does not reveal severe glare problems that may occur infrequently over the year. Because an annual average may be misleading, a weighted glare index was developed to emphasize the fairly infrequent periods of perceptible or uncomfortable glare that occur (see Appendix A). Figure 2-40 shows the weighted glare index for nine glazings in a south-facing office in Chicago with moderately sized windows (WWR=0.30). The weighted glare index varies considerably depending on orientation, the presence of shading devices, and other window properties and design conditions. The weighted glare index is used as the basis to compare windows on a 0-10 scale in later chapters.
